1. Field of the Invention
The present invention relates to a flow rate controller that is employed in fluid transport pipes for various industrial fields, for example, chemical factories, semiconductor manufacturing, food processing, biotechnology, and so on.
2. Description of Related Art
Conventionally, differential pressure flow meters, which measure the flow rate of fluid, have been widely employed. Such a differential pressure flow meter is provided with pressure sensors before and after an orifice and is employed installed at a location for measuring the fluid pressure of fluid containing corrosive chemicals, such as high purity nitric acid, hydrochloric acid, and hydrofluoric acid, for example, as in semiconductor manufacturing processes. To give a specific example, in the case of semiconductor manufacturing apparatuses, a reagent containing hydrofluoric acid is used in etching of a semiconductor substrate, and therefore, a differential pressure flow meter is installed in a circulation circuit of the reagent in order to stably supply the reagent.
In other words, by providing a pair of pressure sensors on the upstream side and the downstream side of an orifice provided in a main fluid channel, the above-described differential pressure flow meter can be used as a flow rate meter that calculates a flow rate by converting differential pressure between the two pressure sensors. In addition, by controlling the degree of opening of an adjustable valve such that there is no difference between the calculated flow rate from the flow rate meter and a preset flow rate set in advance, the flow rate meter serves as a flow rate controller that performs feedback control of the flow rate in the main fluid channel to a desired value (for example, refer to Japanese Unexamined Patent Application, Publication No. Hei 5-233068 (see FIG. 1)).
With a flow rate controller provided with the above-described differential pressure flow meter, as a condition for carrying out accurate flow rate measurement and flow rate control, it is important to the orifice at a constant degree of opening needed to measure a differential pressure.
For example, when dealing with slurry liquid, which coagulates easily, a coagulate produced by coagulation of the liquid may become attached to the orifice, thus narrowing the orifice diameter. Such coagulate attachment is the cause of an error in the measured flow rate, and therefore, it is desirable that orifice clogging, wherein a foreign object such as a coagulate is attached to an orifice part, be detected so as to enable a quick response.
Given such a background, with a flow rate controller using a differential pressure flow meter, it is desirable that the flow rate be controlled by accurately detecting the flow rate with the differential pressure flow meter and by easily detecting a clogged state, wherein a foreign object such as a coagulate is attached to an orifice of the differential pressure flow meter.